Vibration, rolling and pitching will tend to move the engine from its original set position.
The bedplate is aligned and individual chocks are used to maintain that alignement. Holding down studs which are preferred to bolts because they are cheaper are then used to hold the engine firmly to the tank top.
The bedplate must be as rigid as possible in order to keep the crankshaft straight. The ships structure, which includes the tank tops will distort as the ship rolls and pitches. Modern bulk carriers are constructed of high tensile steel because thinner plates may be used, thus reducing the deadweight and allowing more cargo to be carried. Thinner plates are more flexible and this increases the relative movement between tank top and bedplate.
This relative movement between the rigid bedplate and flexing tanktop leads to bending of the stud and possible fatigue failure.
One solution is to use spherical washes but these are expensive. A better solution is to use longer studs. When these bend the radius of curvature is grater and so reducing the bending moment for the same relative movement. A large radius of curvature means a lower stress and the risk of fatigue failure is reduced.
M /I = s/y = E/R
M=Bending moment
I=2nd moment of area of the cross section
s =Stress
y=distance from the axis of bending to the outer face
I=2nd moment of area of the cross section
s =Stress
y=distance from the axis of bending to the outer face
E= modulus of elasticity
R-radius of curvature of the bending.
R-radius of curvature of the bending.
This can be arranged into
s = E . y /R
or s = k. 1 /R
where k is a constant
that is, stress is inversely proportional to the radius of curvature.
